We propose to study the specificity, nature, distribution and significance of the antigen(s) in human mammary cells which cross-react with antibody to mouse mammary tumor virus. We will determine the stability of the antigen(s) in mammary epithelial cells cultured from breast tissues and breast fluid aspirates. Expression of the antigen(s) will be correlated with staging of the breast disease and prognosis and history of the tissue donor. We will attempt to amplify the antigen by manipulation of nutritional milieu, cell interactions and substrate composition with the ultimate objective of biochemical analysis of the antigen.